Gas hydrate occurrences and their relation to host sediment properties: Results from Second Ulleung Basin Gas Hydrate Drilling Expedition, East Sea

The Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) recovered various forms of gas-hydrate bearing sediments from 10 drill sites in the lower slope and basin floor of the Ulleung Basin. To characterize the gas-hydrate occurrences and the properties of the host sediments, whole-round core samples were taken from portions of recovered cores determined to be hydrate-bearing based on infrared (IR) scanning. These samples were further characterized by a variety of shipboard experiments such as imaging of the sediments with hand-held IR and visual cameras, measurements of pore water chlorinity within and around IR inferred cold regions in the core and grain-size analysis of pore-water squeeze cakes. Sediment compositions of selected samples were further characterized by X-ray diffraction and scanning electron microscopes during post-cruise analysis. The shipboard and post-cruise analysis results collectively indicate that the recovered gas hydrates mainly occur as 1) “pore-filling” type bounded by discrete silty sand to sandy silt layers, 2) “fracture-filling” veins and nodules, or 3) “disseminated” type in silt. In addition, minor but significant variation in gas hydrate concentrations were observed in diatomaceous silt where gas hydrates occur as “pore-filling” material in layers dominated by intact diatom frustules. Gas hydrate accumulations of “fracture-filling” type occur predominantly in regions where acoustic blanking features in the seismic record suggest gas migration from below the gas hydrate stability zone. Results from the UBGH2 core studies along with the analysis of similar samples from other expeditions, including those executed by the Ocean Drilling Program, the Integrated Ocean Drilling Program, and the First Ulleung Basin Gas Hydrate Drilling Expedition, greatly improved our understanding of lithologic controls on marine gas hydrate occurrences.